The present invention relates to a gearbox, and particularly an automatic stepless gearbox for motor vehicles, comprising:
a continuous transmission-ratio variator,
sensor means for supplying electrical signals indicative of the speed of the motor vehicle and of the number of revolutions of the engine, and
a control and operating unit for causing, through actuator devices, the variation of the transmission ratio by the variator in accordance with a predetermined mode dependent upon the signals supplied by the sensor means.
The known devices of this type are generally characterised by a satisfactory operation. Under certain conditions of running of the motor vehicle, however, they suffer from drawbacks which cannot be ignored. In general terms, these drawbacks are associated, paradoxically, with the characteristics of full automation of the operation of such devices. Thus, for example, when a motor vehicle fitted with an automatic stepless gearbox of the aforesaid type is travelling down a hill of considerable gradient, the transmission ratio automatically set by the gearbox becomes excessively "long" and the braking action which can be positively and conveniently exerted by the engine of the motor vehicle is absent or inadequate.